Shallow diode arrays made by diffusion or epitaxy techniques on the surface of a silicon wafer are currently used for imaging in the visible region (0.2-0.7 micron). In this wavelength range, the absorption constant, .mu., of silicon is high (10.sup.5 cm.sup.-1) so that 99% of the visible spectrum is absorbed in a shallow layer 0.46 micron thick contiguous to the incident surface. Thus shallow diodes extending to this depth will detect most of the electron-hole pairs generated by the visible light incident on the surface of a shallow diode array.
The high sensitivity of silicon vidicon targets in the visible region does not extend to X-ray and infrared (IR) radiation because of the low absorption constant of silicon for these types of radiation.
To achieve a high sensitivity for X-rays and IR, a thick silicon wafer can be used to compensate for the low absorption constant of silicon. With shallow diode arrays, however, a loss of resolution occurs in thick targets because electrons and holes diffuse out parallel to the surface of the target before they reach the shallow surface diodes from generation points deep in the target bulk. Moreover, because of the limited diffusion length of holes and electrons in silicon (150 microns in silicon with a minority carrier lifetime of 10 microseconds), many holes and electrons recombine before they ever reach the shallow surface diodes.